answer is 53 the elements which have electrons in f orbital are 6 th and 7 th period elements we know that 6 th period consist of 32 elements, while 7 th period consists of 26 elements (refer the periodic table,to answer these type of questions one must be thorough with periodic table) 6 th period consist of lanthanide series which are 14 in number (from at.no 58 to at.no 71) apart from these there are 18 elements in s,p,d orbital from at.no 55 to at.no 86. in these,elements of at.no 55,56,57 do not contain electron in f orbital. therefore the no of elements having electrons in f orbital in 6 th period will be 32-3=29 7 th period consists of 26 elements in which 14 are actinides (from at .no 90 to 103), and remaining are from at .no 87 to 112.we need till 110 only so 26-2=24 elements in 7 th period 29+24=53 elements contain electrons in f orbital
